1. Field of the Invention
The present invention relates to a new class of phosphonates and related salts, the method of preparation of the same and their utilization in the preparation of water additives to be used in different industrial fields. More specifically, the products and the processes according to the present invention provide new additives that prevent the segregation of solids from their aqueous solutions or dispersions by acting as precipitation inhibitors and dispersants.
2. Description of Related Art
Water in its natural state, as found in rivers, lakes and seas, and with the exeception of rain water, contains a certain quantity of metal ions and anions of different types and in various proportions, according to their origins. Such metal ions cause the formation of a precipitate, when water taken from its natural environment is used for industrial purposes. In industrial processes, water which is normally in equilibrium with the external environment is affected by different physical-chemical conditions, and if the concentration of salts under these new conditions exceeds the solubility product (“supersaturation”), salt precipitation is observed.
Such precipitating salts are generally formed by earth-alkali metals (Ca; Ba; Mg); among them Calcium—mostly as carbonate but also as sulfate—is mostly responsible for phenomena of incrustation in several industrial water applications.
The incrustation (not only limited to poory soluble salts) is generally called “SCALE” by water treatment experts.
Several factors cause supersaturation and thus the precipitation of aqueous solutions containing calcium carbonate. The CaCO3/CO2/H2O system is described schematically here in FIG. 1.
Calcium is present in all surface waters in the form of soluble bicarbonate (HCO3−) due to the absorption of carbon dioxide from the atmosphere. Any modification of such a system leads, in a more or less marked way, to precipitation of CaCO3.
The causes for the precipitation of CaCO3 can be classified as follows:
1. Concentration of the solution (evaporation of the aqueous phase);
2. Variations of temperature. By heating, the following transformation takes place:Ca(HCO3)→CaCO3+CO2+H2O
3. Variations of the pH. An increase in the pH of the system results in the following transformation:Ca(HCO3)2+2OH−→CaCO3+CO3−−+2H2O
As far as cooling and/or heat-exchanger circuits are concerned, the incrustation (scale) formation mechanism can be attributed to a precipitation of salts from supersaturated solutions in the regions adjacent to the heat exchange surface of the system.
The effects of such uncontrolled precipitation are sometimes disastrous. For example, in cooling systems, where large volumes of water are used, the deposits of CaCO3 accumulate in large quantity in the pipes, causing a reduction of the thermal exchange capacity and leading to a virtual occlusion of the pipes, making it necessary to remove the deposits by acidic treatment with consequent shutdown of the plant.
Moreover, the formation of a CaCO3 incrustation facilitates the incorporation of solid particles that cannot be chemically removed (e.g. SiO2) or the growth of bacteria and algae.
In order to overcome these disadvantages, pretreatments have been proposed in the prior art that provide for the preventive elimination of low-solubility salts by ionic exchange, precipitation, or by the use of suitable “sequestering agents” and suitable “scale inhibitors”.
Preventive elimination is in most cases not economically acceptable because of the large volumes of water involved.
The same can be said for the chelating agents; it is well known that these substances form water-soluble complexes with the metal ions within a well defined stoichiometric molar ratio.
The preferred treatment in the prior art involves the use of suitable “scale inhibitors” that take advantage of the so called “Threshold Effect.” The Threshold Effect was discovered by observing the behavior of inorganic polyphosphates that prevent the precipitation of the CaCO3 from supersaturated solutions by means of sub-stoichiometric concentrations (Hatch and Rice, Indust. Eng. Chem., 31, 51–53 (1939); Reitemeier and Buehrer, J. Phys. Chem., 44 (5), 535–536 (1940); Fink and Richardson U.S. Pat. No. 2,358,222; and Hatch, U.S. Pat. No. 2,539,305).
The mechanism by which precipitation is inhibited is not completely understood today, although the absorption of an inhibitor onto the crystalline surface seems necessarily to be the first step in the inhibition process. The molecules of the inhibitor are attracted on the growing crystalline surface by the presence of metal cations such as Ca, Mg, Ba, for which they have a great affinity.
Once the molecules of the inhibitor are adsorbed, such molecules reside on the surface of the crystal, thus disturbing the regularity of its growth.
If all of this happens during the “nucleation” phase, i.e. during the stage in which a certain number of molecules in solution begins to aggregate in order to give rise to a crystal nucleus, the inhibitor can disturb nuclear growth to such an extent as to make the nucleus redissolve.
Such ability, exercised by various polyelectrolytes, is particularly marked in the case of phosphonates, which moreover combine corrosion inhibition functions with great resistance to hydrolysis.
However, for every operating condition, there is a limit to the molar ratio between inhibitor and metal. In fact, by increasing the phosphonate quantity beyond a certain limit, precipitation of insoluble calcium salts of phosphonates is observed; in such “turbidity” zone, the phosphonate is no longer active. The effectiveness of the phosphonates at various inhibitor/metal ratios is shown schematically in FIG. 2, wherein the x-axis relates to the molar ratio between metal and inhibitor, while the y-axis relates to the turbidity measured nephelometrically.
Furthermore, it is well known that it is necessary to provide for more effective recovery cycles of industrial water—above all because of an increasing use of water resources—in order to reduce both the quantity of water used and the environmental impact of the treatment agents.